Please help me get a better understanding of PV disconnect use.

[48Rob]

Non grid tie system


I have a PV disconnect switch just before the PV wires enter the MPPT.

I also have a battery disconnect just after the main fuse coming from the battery bank which isolates my batteries from the distribution center.

It is safe to turn off power to my distribution center via the battery disconnect at any time because it is considered low voltage (14 volts) BUT this won’t turn off power coming from the MPPT, that also feeds the distribution center, when the panels are producing.

It is not wise to turn off the PV disconnect while energized because of the arc danger from the much higher voltage (158 volts) except in an emergency.

The outgoing voltage, from the MPPT to the batteries via the distribution center is 14 volts

Placing a second “battery disconnect” between the MPPT and the distribution center, on the 14 volt lines, would allow isolation of the distribution center from both power sources, rendering it safe to change a fuse, rewire a circuit, etc. without causing an arc?

Would doing so damage the MPPT by allowing it to convert the PV voltage, and then blocking the path of the outgoing voltage?

If so, then except for emergency shutdown, I must either cover all the panels, or wait until dark before disconnecting the PV voltage to de-energize the distribution center?

 

[sunshine_eggo]

It is not wise to turn off the PV disconnect while energized because of the arc danger from the much higher voltage (158 volts) except in an emergency.

This statement implies you have a substandard disconnect. A suitable PV disconnect should either carry little risk of arcing, or be designed to deal with it.

The outgoing voltage, from the MPPT to the batteries via the distribution center is 14 volts

Placing a second “battery disconnect” between the MPPT and the distribution center, on the 14 volt lines, would allow isolation of the distribution center from both power sources, rendering it safe to change a fuse, rewire a circuit, etc. without causing an arc?

Yes. It's not uncommon for folks to install a suitably rated breaker for over-current protection, which I hope you already have (fuse or breaker), and as a disconnect.

Would doing so damage the MPPT by allowing it to convert the PV voltage, and then blocking the path of the outgoing voltage?

Potentially. It depends on the MPPT brand and the circumstances at the time of disconnect. You absolutely, positively will get a voltage spike when the "load" (battery or actual loads) are removed from the MPPT.

If so, then except for emergency shutdown, I must either cover all the panels, or wait until dark before disconnecting the PV voltage to de-energize the distribution center?

Nobody does this. Your position sounds like someone has warned you about issues that aren't really a thing, or are overblown.

 

[12vDC]

IIRC I read in my MPPT manual that the battery must be connected prior to connecting the panel to the controller.

I take that to also mean you must disconnect the panel prior to disengaging the battery.

I use a Midnite Solar breaker off the panel (70Voc) to the mppt and a Mechanical Products breaker from mppt to the battery.

 

[zanydroid]

It is not wise to turn off the PV disconnect while energized because of the arc danger from the much higher voltage (158 volts) except in an emergency.

If your disconnect is not switch rated then you need to turn off as many loads as possible to reduce the current going through it.

 

[48Rob]

Sounds like I was misinformed, and should have placed a question mark behind my statements.
I do have a good disconnect switch, but was under the impression that it wasn’t supposed to be turned off under load.
I understand how to remove loads using the battery disconnect switch, but believed that power coming from the MPPT was also a “load".

Thanks all, for setting me straight!

 

[zanydroid]

I do have a good disconnect switch, but was under the impression that it was’t supposed to be turned off under load.

You shouldn’t turn it off under load. Can’t the MPPT be turned off first?

 

[48Rob]

You shouldn’t turn it off under load. Can’t the MPPT be turned off first?

Okay, I think I have it now.

First remove as many loads from the system as possible.
Then the PV disconnect is switched to "off" first, then the DC disconnect is switched to "off".
It doesn't hurt the PV switch to be turned off if sun is shining on the panels, but it can be hard on the DC switch (arcing) if I don't kill the incoming power the solar panels is feeding the MPPT, and batteries first.?

 

[zanydroid]

Yes if sun is shining and load drawn through the panels you don’t want to switch anything not rated to switch load.

I think you want to turn off the MPPT. Does it not have a switch or app to do that? Because how else are you going to stop the battery from drawing charge current?

 

[12vDC]

Okay, I think I have it now.

First remove as many loads from the system as possible.

yes

Then the PV disconnect is switched to "off" first, then the DC disconnect is switched to "off".

yes

It doesn't hurt the PV switch to be turned off if sun is shining on the panels, but it can be hard on the DC switch (arcing) if I don't kill the incoming power the solar panels is feeding the MPPT, and batteries first.?

interrupting the circuit from the PV will disengage the controller = 0V 0A

if the controller is converting power but is disengaged from the battery there is nowhere for that converted power to go. the controller will burn out.

as intimated above in a previous response, your concern for the PV disconnect arcing is more likely if you are utilizing an uncertified unproven design breaker (amazon chynesium). not so much in the proper procedure to disconnect.

 

[zanydroid]

as intimated above in a previous response, your concern for the PV disconnect arcing is more likely if you are utilizing an uncertified unproven design breaker (amazon chynesium). not so much in the proper procedure to disconnect.

Some UL listed disconnects are not rated to interrupt under load. I doubt you want to turn off an IMO under load.

A UL listed breaker probably can.

Actually if you don’t have to exercise the arc extinguishing feature by following the correct procedure , that’s probably better. You have to write down the startup procedure anyway

 

[48Rob]

This is the PV disconnect I plan to use.
I am using a Victron controller, so will read through the manual to see the suggested shut down procedure.

Thank you for the discussion and ideas.

 

[12vDC]

Pulled this off the web somewhere:

“MidNite's breakers are rated to break the full rated load at the rated voltage repeatedly, with NO DAMAGE. Always use a properly sized breaker for disconnecting.

MidNite breakers have been evaluated by ETL to 150VDC and are listed for US and Canada. All of the NRTL listings still apply.”


For my degree as a Sidewalk Mechanic I feel this is safe for my application. Your mileage and circumstance may vary as apparently the internet states you should NEVER disconnect a panel while it is making electricity.

??‍

 

[zanydroid]

Looks like IMO says their disconnects are rated for switching under load. But I would tag someone that really knows these products and can provide a direct reference (like screen shot and page number level on official documentation) for your country before going by that.

 

[48Rob]

Thank you.